drm/i915/skl: Remove spurious warn in get_ddi_pll()

When reading out a DDI config that uses a PLL that is not part of the
shared_dpll scheme (DPLL0), it's totally normal to end up in the
default: case of that switch.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>